Add Twitter Card support for subreddit listings

More pretties more places.

As with the previous Twitter Card work (1c6df70), this relies heavily
upon the OpenGraph work done earlier in 726d9be.

r2/r2/controllers/listingcontroller.py

@@ -241,14 +241,23 @@ def GET_listing(self, **env):
 class SubredditListingController(ListingController):
     private_referrer = False
 
-    def _build_og_title(self):
+    def _build_og_title(self, max_length=256):
         sr_fragment = "/r/" + c.site.name
         title = c.site.title.strip()
         if not title:
-            return sr_fragment
+            return trunc_string(sr_fragment, max_length)
 
         if sr_fragment in title:
-            return _force_unicode(title)
+            return _force_unicode(trunc_string(title, max_length))
+
+        # We'd like to always show the whole subreddit name, so let's
+        # truncate the title while still ensuring the entire thing is under
+        # the limit.
+        # This doesn't handle `max_length`s shorter than `sr_fragment`.
+        # Unknown what the behavior should be, but realistically it shouldn't
+        # happen, since this is scoped pretty small.
+        max_title_length = max_length - len(u" • %s" % sr_fragment)
+        title = trunc_string(title, max_title_length)
 
         return u"%s • %s" % (_force_unicode(title), sr_fragment)
 
@@ -264,13 +273,22 @@ def render_params(self):
             return {'show_locationbar': True}
         else:
             if not c.user_is_loggedin:
+                # This data is only for scrapers, which shouldn't be logged in.
                 return {
                     "og_data": {
                         "site_name": "reddit",
                         "title": self._build_og_title(),
                         "image": static('icon.png'),
                         "description": self._build_og_description(),
-                    }
+                    },
+                    "twitter_card": {
+                        "site": "reddit",
+                        "card": "summary",
+                        "title": self._build_og_title(max_length=70),
+                        # Twitter will fall back to any defined OpenGraph
+                        # attributes, so we don't need to define
+                        # 'twitter:image' or 'twitter:description'.
+                    },
                 }
 
             return {}